If driving, the halfway point is in Salt Lake City, Utah, with about 341 miles to Boise and 341 miles to Montrose. As the bird flies, the halfway point is just north of Salt Lake City, around Layton or Bountiful.
lincoln, nebraska
The halfway point is basically the area where Nebraska, Colorado and Wyoming meet. This is not an exact answer, but in relative terms this is the halfway point between Salmon, Idaho and Kansas City, Missouri. If your looking for a town, then I would say the capitol city of Wyoming (Cheyenne) is pretty much as close as your going to get to a city halfway between Salmon and K.C.
Taylor, Idaho is the halfway point.
North Dakota
Between Colorado and Idaho, Idaho is the state that is farthest west in the United States. It is also father west than Wyoming, New Mexico and Montana.
between north and northwest
salt lake city
La Grande, OR
Erie, CO
Gretna, NE
Pendleton, Oregon, seems to be about halfway between Boise and Tacoma. The Tri-Cities are nearby the halfway point, and they are a larger metro area.
The halfway point when driving is Lexington, Nebraska, exit 237 off I-80.